In addition to neutralization, antibodies can mediate a variety of Fc-dependent effector functions and defining the role of these in vivo could be critical to determine the antibody response or combination of responses that offer the most optimal protection against HIV exposure. Using the SHIV/macaque model and the broadly neutralizing human lgG1 b12 antibody, we have demonstrated that interaction with Fc gamma receptors (FcyRs) plays an important role in IgG-mediated protection. We have investigated the specificity of this interaction in more detail and recently showed that FcYRIila-mediated ADCC may be less important than commonly thought. In ongoing studies, we are performing an in vivo investigation of the role of FcyRiia- mediated phagocytosis in antibody protection and are confident that we will soon have a clear picture of which FcyRs and corresponding effector functions contribute to lgG1 b12-mediated protection. To expand our investigation and gain a quantitative assessment of effector function contribution to protection, we propose to investigate additional antibody specificities and isotypes in a fully primatized model. The specific aims are: (1) To determine the magnitude and nature of effector function contribution to protection against mucosal SHIV challenge by the highly potent broadly neutralizing antibody PGT121 as a fully primatized antibody, (2) To determine the importance of antibody isotype in protection against mucosal SHIV challenge by comparing topically administered primatized secretory IgA (SIgA) PGT121 to IgG PGT121, and (3) To correlate neutralization, extra-neutralizing properties and epitope availability with in vivo protection for neutralizing and non-neutralizing antibodies. RELEVANCE (See instructions): Understanding antibody-mediated protection against HIV is crucial for vaccine design. The significance of this project is that it allows for a direct investigation of the antibody properties that contribute to protection in vivo. Neutralization is a well-established and highly desirable property, but accumulating evidence strongly suggests that Fc-mediated effector functions also play an important role in antibody-mediated protection in vivo. In vaccine design, the main problem has been creating immunogens that elicit broadly neutralizing antibodies and many laboratories, including our own, have efforts in this area. If we can correlate protection with in vitro assays measuring Fc-mediated effector function, perhaps in addition to neutralization, we would then be able to determine the type of antibody responses that will provide greatest benefit in the context of a vaccine.